Uncoupling cell division and cytokinesis during germline development in metazoans

Abstract

The canonical eukaryotic cell cycle ends with cytokinesis, which physically divides the mother cell in two and allows the cycle to resume in the newly individualized daughter cells. However, during germline development in nearly all metazoans, dividing germ cells undergo incomplete cytokinesis and germ cells stay connected by intercellular bridges which allow the exchange of cytoplasm and organelles between cells. The near ubiquity of incomplete cytokinesis in animal germ lines suggests that this is an ancient feature that is fundamental for the development and function of this tissue. While cytokinesis has been studied for several decades, the mechanisms that enable regulated incomplete cytokinesis in germ cells are only beginning to emerge. Here we review the current knowledge on the regulation of germ cell intercellular bridge formation, focusing on findings made using mouse, Drosophila melanogaster and Caenorhabditis elegans as experimental systems.

Keywords: germ cells; germline development; incomplete cytokinesis; intercellular bridges; metazoan.

FIGURE 1

Germline syncytia vary in architecture. (A) Germline syncytia formed by intercellular bridges that directly connect germ cells. These syncytia can be linear (upper) or branched (lower), depending on the number of bridges per cell. (B) Germline syncytia formed by cytoplasmic bridges that connect germ cells to a common cytoplasmic core.

FIGURE 2

Intercellular bridges during spermatogenesis in the mouse testis. (A) Spermatogenesis occurs within the convoluted seminiferous tubules and is polarized, with mitotic spermatogonia at the basal surface and maturing spermatids at the luminal surface. (B) Both mitotic and meiotic divisions during spermatogenesis are incomplete and germ cells form long synchronous chains until individuation as mature sperm. (C) An electron micrograph showing a stable intercellular bridge (arrowheads) between two spermatocytes in the rat testis. Reproduced with permission from Dym and Fawcett, 1971. (D) A cross-section of a mouse seminiferous tubule stained for TEX14 to mark stable intercellular bridges (red), actin (green) and nuclei (DAPI, blue). Intercellular bridges are evident throughout the tubule. Arrow indicates the lumen. Scale bar is 25 μm. (E) A higher magnification view showing TEX14-labelled intercellular bridges (red; arrows) between spermatogonia (green) in the mouse testis. Scale bar is 10 μm. (D) and (E) were adapted with permission from Greenbaum et al., 2006, Copyright (2006) National Academy of Sciences, United States.

FIGURE 3

Ring canals during oogenesis in the Drosophila ovary. (A) Each ovary in female Drosophila contains 16–23 ovarioles. (B) Oogenesis is polarized along the length of each ovariole, with germline stem cells (GSCs) and mitotic cystocytes in the anterior germarium, maturing egg chambers arrayed along the vitellarium, and mature eggs (stage 14) at the posterior. (C) Cystocyte divisions are incomplete, giving rise to a 16-cell cyst that is maximally branched, with cystocytes connected by stable intercellular bridges called ring canals. The oocyte develops from one of the two cystocytes with the most ring canals. The remaining cells differentiate as nurse cells. (D) An electron micrograph showing a ring canal between two nurse cells. The ring canal forms a break in the plasma membrane (pm) and is composed of an inner (ir) and outer (or) rim. (E) A stage 10 egg chamber with ring canals immunostained for hu-li tai shao (Hts). The oocyte is indicated with an asterisk. Insert shows the actin staining pattern for a ring canal from an egg chamber of the same stage. (D) and (E) adapted with permission from Robinson et al., 1994, The Company of Biologists, Ltd.

FIGURE 4

Rachis bridges in the C. elegans hermaphrodite gonad. (A) The C. elegans hermaphrodite gonad is composed of two U-shaped tubes. Mitotic germ cells at the distal tip of each arm enter meiosis as they move proximally, and maturing oocytes are found at the proximal tip, adjacent to the spermatheca. (B) Germ cells are arranged in a rough monolayer around a common core of cytoplasm called the rachis. Each germ cell maintains a stable rachis bridge connecting it to the rachis. (C) An electron micrograph of a cross section through the gonad showing a germ cells (gc) with an open rachis bridge. Somatic sheath cells (sh) contact germ cells basally. Scale bar is 5 μm. Adapted with permission from Hall et al., 1999, Elsevier. (D) A maximum intensity projection of ∼half of the adult gonad tube, showing the rachis surface (green; marked by ANI-2:GFP) and cell membranes (magenta; marked by mCherry:PH^PLCδ). (E) A longitudinal section of the adult gonad showing the rachis bridges (green; marked by ANI-2:GFP) connecting each germ cell to the rachis. Cell membranes are in magenta. (D) and (E) are adapted from Priti et al., 2018, CC-BY. Scale bars are 10 μm. (F) All germ cells are derived from the P₄ germline precursor cell that gives rise to the two primordial germ cells (PGCs) during embryogenesis. P₄ undergoes incomplete cytokinesis and the two embryonic PGCs remain connected by a stable intercellular bridge. By an unknown mechanism, the connection between the two PGCs is transformed into the rachis primordium during late embryogenesis, such that, by the time the L1 larva hatches, each PGC possesses its own rachis bridge.

FIGURE 5

Stable intercellular bridge formation in germ cells by incomplete cytokinesis. In metazoans, stable intercellular bridges (orange) arise from the cytokinetic/midbody ring (magenta) following incomplete germ cell cytokinesis, through mechanisms that impede the activity of ESCRT components during abscission. These events are reiterated in germ cells that develop within linear and branched cysts (e.g., mouse, Drosophila), thus enabling cyst expansion. In architectures where germ cells are connected to an anucleate cytoplasmic core (e.g., C. elegans), the stable intercellular bridges must undergo duplication through a mechanism that is currently unknown. We propose that subsequent germ cell divisions within this architecture also occur by incomplete cytokinesis, in a manner that allows one daughter cell to inherit the pre-existing stable intercellular bridge while the other daughter inherits the bridge arising of the cytokinetic/midbody ring. This mechanism requires further experimental validation and could also rely on bisection of the stable intercellular bridge by the cytokinetic ring, as was proposed previously (Świątek et al., 2009).